Sialyl-oligosaccharides are of growing importance due to their function as ligands of selectins [(a) Phillips et al., Science, 250:1130 (1990); (b) Lowe et al., Cell, 63:475 (1990); (c) Lasky, Science, 258:964 (1992)] and as important recognition elements on the surface of a variety of cells, [Magnani et al., Cancer Res., 46:5489 (1983)]. Enzymatic approaches to the synthesis of these compounds have proven to be useful, [(a) Ichikawa et al., Anal. Biochem., 202:215 (1992); (b) Toone et al., Tetrahedron, 45:5365 (1989); (c) David et al., Adv. Carbohydr. Chem. Biochem., 49:175 (1991)] especially that based on glycosyltransferases coupled with regeneration in situ of sugar nucleotides. Wong et al., J. Org. Chem., 47:5416 (1982); Wong et al., J. Org. Chem., 57:4343 (1992); Auge et al., Carbohydr. Res., 151:147 (1986); Thiem et al., Angew. Chem. Int. Ed. Engl., 30:1164 (1991); Thiem et al., Synthesis, 141 (1992); Ichikawa et al., J. Am. Chem. Soc., 113:4768 (1991); Ichikawa et al., J. Am. Chem. Soc., 113:6300 (1991); Ichikawa et al., J. Am. Chem. Soc., 114:9283 (1992). By using galactosyltransferase and sialyltransferase, sialyl N-acetyllactosamine, for example, was obtained in moderate yields. Ichikawa et al., J. Am. Chem. Soc., 113:6300 (1991).
Glycosidases have also been utilized in glycosylation. The yields, however, are relatively low and the regioselectivity is often difficult to control, [(a) Nilsson et al., TIBTECH; 6:256 (1988); (b) Kobayashi et al., J. Am. Chem. Soc., 113:3079 (1991); (c) Chaplin et al., J. Chem. Soc. Perkin Trans., 1:235 (1992); (d) Look et al., Tetrahedron Lett., 33:4253 (1992); (e) Sakai et al., J. Carbohyd. Chem., 11:553 (1992); (f) Vanderkerckhove et al., Glycobiology, 2:541 (1992); (g) Kitahata et al., Agric. Biol. Chem., 55:2433 (1991)] although certain glycosidases catalyze regioselective transglycosylation. Sakai et al., J. Carbohyd. Chem., 11:553 (1992); Vanderkerckhove et al., Glycobiology, 2:541 (1992).
Another problem with the use of glycosidases is that the product of the reaction is subject to the enzymatic cleavage, making the process difficult to control. Thus, the product for the glycosyl transfer reaction is itself a substrate for the enzyme and is cleaved.
For example, Nilsson, Carbohyd. Res., 188:9-17 (1989) reported yields of only about 10-35 percent using a .beta.-galactosidase in a transglycosylation. Some isolated products made by such transglycosylations were also separately sialylated using CMP-Neu5Ac and a .alpha.(2,3)sialyltransferase.